Intrathyroidal Concentrations of Methimazole in Patients with Graves’ Disease*

A sensitive gas chromatographic-mass spectrometric method enabled us to study intrathyroidal concentrations of methimazole in 20 euthyroid patients with Graves' disease on treatment with carbimazole and T4. There was no difference between patients receiving a final dose of carbimazole (10 mg), known to be totally bioactivated to methimazole (6.1 mg), 3-6 h before thyroid excision (518 ng/g thyroid tissue +/- 90 SEM) and patients who received the same dose 17-20 h before excision (727 ng/g thyroid tissue +/- 157 SEM), indicating a slow intrathyroidal turnover of the drug. On the other hand, the serum concentrations were much higher in the first group (102 ng/ml +/- 5 SEM vs. 16 ng/ml +/- 3 SEM), reflecting a short plasma half-life of the drug. The intrathyroidal concentrations of methimazole ranged from 230-1895 ng/g among individual glands but were similar in pieces from different parts of a single gland. These methimazole concentrations are lower than the concentrations reported by others to have an immunosuppressive action on lymphocytes in vitro.